A vehicle constant speed control system including a vehicle speed detector and a controller for controlling the engine throttle valve so that the engine output is controlled to make the vehicle speed approach a target speed. The controller compares the actual vehicle speed with the target speed and calculates a required driving torque for driving the vehicle at the target speed and a target throttle valve position which is required for producing the required driving torque. The controller then produces a driving signal to actuate the engine throttle valve so that the required driving torque is produced and the target speed is attained.